I can still add devices from my IQ4 panel but pretty inconvenient when I’m 3 stories upstairs adding a switch. So far I have 13 zwave light switches and 2 zwave locks so don’t think I’m at any limit.
Standard Z-wave devices have a limited range, it is a mesh network which relies on AC powered devices in the network to extend that range. You may just be running into a distance issue with these new devices.
As the other post suggests, you may instead need to bring the panel close to the device, learn it in, then run a network rediscovery when your panel is back in its permanent spot.
The panel’s Power Management setting must be disabled to do this so that Z-wave menus are accessible on battery only.
This is under Settings > Advanced Settings > Installer code > Installation > Installer Settings.